This invention relates to threaded fasteners and more particularly relates to corrosion resistant drill screws.
Threaded fasteners such as typical drill screws are widely used for example in automotive applications in which the screws are driven directly into sheet metal and similar panels for attachment of a variety of fixtures. Particularly in such automotive applications corrosion resistance is an important requirement of such drill screws. As is well known, zinc-plated, galvanized steel screws have been conventionally employed for drill screw applications requiring such corrosion resistance. For similar applications, aluminum plated steel drill screws have more recently been developed and have additionally been provided with decorative and protective, pigmented top coatings such as polymeric paints.
It has been known, as described for example in Australian Pat. No. 73,297 and in U.S. Pat. No. 3,438,754, that zinc-plated steel articles such as structural sheets and panels can be improved in corrosion resistance by applying an aluminum overlay upon the zinc plating. The improved corrosion resistance of these compositely plated steel substrates has been effective for structural panels which are exposed to severely corrosive conditions such as salt spray.
In accordance with this invention, zinc and aluminum plated threaded fasteners not only provide improved corrosion resistance, but in addition provide particularly reduced torque load in driving such fasteners for example in drilling applications, such as in the automobile industry wherein ease and efficiency of drilling along with a significant spread between drilling torque and stripping torque are important fabrication objective.